Today large wind turbines for onshore or offshore use have become so large and heavy that they are manufactured and assembled in sections. A wind turbine today typically comprises a wind turbine tower with a height of at least 50 m, a nacelle coupled to the top of the wind turbine tower in which the generator is located, a rotor hub coupled to the generator via a rotor shaft, and two or three wind turbine blades coupled to the rotor hub. These pieces are typically manufactured separately and transported to the installation site or pre-assembled in units. The pieces or the pre-assembled units are then lifted into position and assembled using one or more cranes at the installation site.
A wind turbine tower is typically manufactured in a number of tower sections which are easier to handle and to transport out to the installation site. The individual pieces of a wind turbine are typically assembled using a large crane and a small crane. The large crane is coupled to the upper part of a tower section while the small crane is coupled to the lower part of that tower section. The two cranes then work together for positioning the latter tower section over a former tower section. Workers then guide the latter tower section, as it is lowered, so that it is positioned correctly on the former tower section. In order to guide the latter tower section into its correct position, the workers have to be situated inside the former tower section at the top. As the latter tower section is lowered, the workers push and pull on the large and heavy structure in order to manoeuvre it into position. This often means that they have to place their body or arm in between the contact surfaces of the two tower sections, thus creating a dangerous situation for the workers as long as the latter tower section hangs freely in the air. As the latter tower section is able to move freely until it is positioned firmly on the former tower section, it may scrape off the sealant located on the contact surface of the former tower section. This also means that the assembly process is not carried out in wind speeds above 10 m/s.
WO 2012/075607 A1 discloses a method of assembling a wind turbine tower which solves this problem by using a system of guide wires extending between two tower sections. Before lifting the latter tower section into position over the former tower section, two guide wires are threaded through a top flange and a bottom flange in the latter tower section and through a top flange in the former tower section. The guide wires are secured to a stop element at the free end and a pulley located on the crane boom ensured that the latter tower section is vertically aligned with the former tower section when the guide wires are tensioned using the winches located on the ground. This enables the workers to be situated outside the wind turbine tower when the latter tower section is lifted and lowered into position on top of the former tower section.
The system of guide wires complicates the assembly process and increases the assembly time as the guide wires first have to be threaded through the respective holes in the flanges in the two tower sections. This increases the risk of the guide wires becoming entangled during the initial phase of the assembly process, and the risk of a misalignment of the two tower sections if the guide wires are threaded through the same opposite facing holes in the respective flanges. Before a new tower section or the nacelle can be lifted and lowered into position on top of the latter tower section, the guide wires first have to be unthreaded and removed from the structure. This added step increases the total assembly time and creates a potential dangerous situation for the workers on the ground if the free end of the guide wires accidentally drops from the structure.
GB 2483677 A discloses a guiding system for guiding the wind turbine blades into position on a rotor hub during the assembly process. The guiding system comprises a substantially L-shaped guiding element mounted to a pitch bearing of the rotor hub. The guiding element is configured to engage a socket-shaped element placed on the inner surface of the wind turbine blade adjacent to the bolts facing the rotor hub. The guiding element is arranged on the outer surface of an inner ring of the pitch bearing while the wind turbine blade is bolted to an intermediate ring of the pitch bearing.
This configuration is not a suitable solution for assembling two tower sections, since the mounting interfaces of such tower sections have a very different configuration. The mounting flanges of the tower sections only comprise a single row of holes used for mounting two adjacent tower sections, no secondary row of holes are provided for the guiding element. The mounting flanges of the tower section extend inwards from the inner surface, thus preventing the guiding element from engaging the socket element.